Method for determining the azimuth of a borehole

ABSTRACT

A method for eliminating the influence of drill string magnetization on an azimuth measurement made by a magnetometer package disposed in the drill string. The method comprises first eliminating the influence of the cross-axial components of the drill string magnetization by taking magnetometer readings at various angular orientations of the drill string and then eliminating the influence of the axial component of the drill string magnetization.

BACKGROUND OF THE INVENTION

The invention relates to a method for determining the azimuth of aborehole that is being drilled in a subsurface earth formation.

The invention relates in particular to a method for determining andcorrecting the influence of the erroneous magnetic field caused bymagnetization of a drill string on an azimuth measurement made by amagnetic sensor package included in the drill string.

During deephole drilling operations it is general practice to surveyfrom time to time the course of the borehole by means of a sensorpackage which is included in the drill string near the lower endthereof. The sensor package generally comprises a set of magnetometersthat measure the components of the local magnetic field in threeorthogonal directions. These measurements together with the direction ofthe earth magnetic field vector, and the direction of the local gravityvector, provide a suitable reference to determine the course of theborehole.

When measuring the orientation of the sensor package relative to theearth magnetic field vector while the drill string is present in theborehole the erroneous magnetic field caused by drill stringmagnetization may cause a significant error in the measurement. Toreduce the magnitude of this error as much as possible it is currentpractice to arrange the sensor package in a drill collar which is madeof non-magnetic material. Moreover, this collar is usually arranged in adrill string section comprising a series of non-magnetic collars toreduce the impact of the steel components of the drilling assembly, suchas the drill bit and the drill pipes above the collars, on the magneticfield at the location of the sensors. A problem encountered when usingnon-magnetic drill collars is that these collars may become magnetizedduring drilling and in particular, the presence of so-called magneticspots in the collar near the sensor assembly may impair the accuracy ofthe azimuth measurement considerably.

U.S. Pat. No. 4,163,324 describes a method for partially eliminating theerror in the azimuth measurement caused by the erroneous magnetic fieldat the location of the sensor package, which field mainly is the resultof drill string magnetization. In the patented method it is assumed thatat the location of the sensors the vector of the erroneous magneticfield is oriented along the borehole axis. Although the known correctionmethod generally enhances the accuracy of the azimuth measurement itdoes not correct for cross-axial magnetic error field. Said cross-axialmagnetic error fields can originate from the presence of magnetic spotsor steel components in the drilling assembly.

SUMMARY OF THE INVENTION

The invention aims to provide an improved azimuth measurement whereinthe error caused by drill string magnetization is corrected for in amore accurate manner than in the prior art method.

In accordance with the invention there is provided a method ofdetermining the influence of drill string magnetization on an azimuthmeasurement in a borehole by means of a sensor package included in adrill string. The package has a central axis z substantially coaxial tothe longitudinal axis of the borehole, and comprises at least onemagnetometer for measuring a cross-axial component of the magnetic fieldB_(m) at the location of the sensor package. The method comprisingeliminating the influence of both the cross-axial and the axialcomponents of the drill string magnetization at the location of themagnetometer. Prior to eliminating the influence of axial drill stringmagnetization the influence of cross-axial drill string magnetization iseliminated by rotating the drill string with the included sensor packageabout the longitudinal axis in the borehole while measuring saidcross-axial component of the magnetic field for various orientations ofthe drill string.

In a preferred embodiment of the invention the sensor package comprisesthree magnetometers for measuring the components B_(x), B_(y) and B_(z)in three mutually orthogonal directions x, y and z, wherein theinfluence of the cross-axial error components M_(x) and M_(y) caused bydrill string magnetization on the measured magnetic field is determiningby plotting, in a diagram having B_(x) as abscis and b_(y) as ordinate,the measured cross axial components B_(x) and B_(y) of the magneticfield at various orientations of the sensor package in the borehole. Ifthe drill string is rotated over an angular interval of about 360degrees a closed spherical curve can be drawn in the diagram through thecross-axial components B_(x) and B_(y) thus measured, whereupon thecross-axial error components M_(x) and M_(y) of the drill stringmagnetization vector M can be determined on the basis of the center ofthe curve in the diagram.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic perspective view of a drill string including atri-axial survey instrument.

FIG. 2 is a diagram in which the cross-axial magnetic field measured bythe cross-axial sensors is plotted while the drill string is rotated inthe borehole.

FIG. 3 is a vector diagram illustrating the position of the vector ofthe measured magnetic field, corrected for cross-axial drill stringmagnetization, relative to a cone defined by the gravity vector and thevector of the earth magnetic field.

FIG. 4 is a diagram in which the distance between the base circle of thecone and said corrected vector is calculated for various assumedmagnitudes of axial drill string magnetization.

FIG. 5 illustrates an alternative embodiment of the invention whereinthe sensor package includes a single magnetometer.

FIG. 6 illustrates the magnetometer readings of the instrument of FIG. 5for various orientations of the instrument obtained by rotating thedrill string.

DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1 there is shown a drilling assembly 1 comprising a drill bit 2which is coupled to the lower end of a drill string 3. The lowermostsection of the drill string 3 includes two non-magnetic drill collars 4.In one of the non-magnetic drill collars 4 a tri-axial survey instrument5 is arranged, which instrument is used to determine the azimuth andinclination of the central axis z of the collar 4, which axis issubstantially coaxial to the longitudinal axis of the borehole at thelocation of the bit 2.

The survey instrument 5 comprises three accelerometers (not shown)arranged to sense components of gravity in three mutually orthogonaldirections x, y and z, and three magnetometers (not shown) arranged tomeasure the magnetic field at the location of the instrument in the samethree mutually orthogonal directions.

In FIG. 1 there is illustrated the gravity vector g measured by theinstrument 5, which vector g equals the vector sum of the componentsg_(x), g_(y) and g_(z) measured by the accelerometers, and the vectorB_(m) of the local magnetic field, which vector B_(m) equals the vectorsum of the components B_(x), B_(y) and B_(z) measured by themagnetometers of the instrument 5. As illustrated the vector B_(m) isoriented at an angle θ_(m) relative to the gravity vector g, which anglecan be calculated on the basis of known mathematical formulas.

In FIG. 1 there is also illustrated the vector B_(o) of the true earthmagnetic field and the dip angle θ_(o) of this vector relative to thegravity vector g. The magnitude of the vector B_(o) and the orientationthereof relative to the gravity vector g can be obtained independentlyfrom the borehole measurement, for example from measurements outside orinside the borehole or from geomagnetic mapping data.

As can be seen in FIG. 1 the measured magnetic field vector B_(m) doesnot coincide with the true magnetic field vector B_(o). This is causedby the erroneous magnetic field M at the location of the instrument,which field is mainly a consequence of the presence of isolated magneticspots S in the non-magnetic drill collars 4 and of the presence of steelcomponents in the drilling assembly 1. In FIG. 1 the vector M_(xy),which cross-axial vector M_(xy) equals the vector sum of the componentsM_(x) and M_(y).

In accordance with the invention the influence of the erroneous magneticfield M is eliminated by first determining the cross-axial vector M_(xy)and then determining the axial component M_(z) of the erroneous field.

Determination of the cross-axial vector M_(xy) is carried out byrotating the drill string over about 360 degrees, thereby rotatingsimultaneously the instrument 5 about the central axis z, whilemeasuring continuously or intermittently the magnetic field B_(m) forvarious orientations of the instrument 5 relative to the central axis z.As illustrated in FIG. 1 rotation of the drilling assembly over 360degrees in the direction of the arrow will cause the vector M_(xy) torotate simultaneously in the same direction, thereby describing a circleC. The magnitude and direction of the vector M_(xy) is determined fromthe plotted diagram, shown in FIG. 2, in which the cross-axialcomponentas B_(x) and B_(y) of the measured magnetic field B_(m) areplotted for various orientations of the instrument relative to thecentral axis z. In the plotted diagram the measured values of B_(x) andB_(y) lie on a circle which is located eccentrically relative to thecenter (0,0) of the diagram. The vector M_(xy) is subsequentlydetermined on the basis of the location of the circle-center 10 relativeto the center (0,0) of the diagram. As illustrated the magnitude of thevector M_(xy) is determined from the distance between the circle-center10 and the center (0,0) of the diagram.

Now a vector B is introduced in the vector diagram of FIG. 1, whichvector B equals B_(m) -M_(xy).

    M.sub.xy =(M.sub.x, M.sub.y, 0)

and

    B.sub.m =(B.sub.x, B.sub.y, B.sub.z)

the vector B can be expressed through

    B=(, B.sub.x, B.sub.y, B.sub.z)-(M.sub.x, M.sub.y, 0).

Defining now the components

    B.sub.x -M.sub.x as B.sub.xc

and

    B.sub.y -M.sub.y as B.sub.yc

gives:

    B=(B.sub.xc, B.sub.yc, B.sub.z)=(B.sub.x, B.sub.y B.sub.z)-(M.sub.x, M.sub.y, 0)                                               (1)

Equation (1) provides a correction for the influence of cross-axialdrill string magnetization on the magnetic field measured by the surveyinstrument 5.

After having thus eliminated the influence of cross-axial drill stringmagnetization M_(xy) on the survey measurement, the influence of theaxial error component M_(z) may be corrected for by a correction methodsimilar to the method disclosed in U.S. Pat. No. 4,163,324.

It is preferred, however, to correct the survey measurement by theinstrument 5 for axial drill string magnetization by means of thecalculation method described hereinbelow with reference to FIG. 3.

The magnitude of the vector B can be expressed by:

    B=(B.sub.xc.sup.2 +B.sub.yc.sup.2 +B.sub.z.sup.2).sup.1/2  (2)

and the magnitude of the gravity vector g by:

    g=(g.sub.x.sup.2 +g.sub.y.sup.2 +g.sub.z.sup.2).sup.1/2    (3)

which enables calculating a dip angle θ between the vectors B and gthrough the formula:

    θ=cos.sup.-1 [(B.sub.xc g.sub.x +B.sub.yc g.sub.y +B.sub.z g.sub.z)/Bg]                                              (4)

The angle θ is indicated in FIG. 1 and also in FIG. 3, which is asimilar but simplified representation of the vector diagram shown inFIG. 1.

Determination of the position of the vector B_(o) relative to the vectorB is complicated by the fact that the vector B is only defined by itsorientation at a dip angle θ relative to the gravity vector g. Moreover,the exact orientation of the true magnetic field vector B_(o) relativeto the axes x, y and z is still unknown. However, as the true magneticfield vector B_(o) is oriented at an angle θ_(o) relative to the gravityvector g it is understood that in the vector diagram of FIG. 3 thevector B_(o) will lie on a cone 12 having a central axis coinciding withthe vector g and a top angle that equals 2θ_(o). The angle θ_(o) isknown as it has been obtained independently from the boreholemeasurement.

Now the distance E is introduced in the vector diagram where E indicatesthe distance between the base circle 13 of the cone 12 and the terminalpoint of the vector B.

The magnitude of the distance E is given by the equation

    E=[B.sup.2 +B.sub.o.sup.2 -2BB.sub.o cos (θ-θ.sub.o)].sup.1/2(5)

The value for E thus found is now plotted in the diagram shown in FIG.4, in which B_(z) is the abscis and E the ordinate.

The next step is to assume that the axial component B_(z) of themagnetic field measured by the instrument 5 may vary as a result of theaxial component M_(z) of the erroneous field. Then various assumedvalues are taken for B_(z) and for each assumed value the correspondingvalue of the distance E is calculated through equations (2), (3), (4)and (5). The various values thus found for E are plotted in the diagramof FIG. 4 which will provide a plotted curve 14 in which at a certainvalue B_(zc) of B_(z) a minimum 15 occurs. The magnitude of the axialcomponent M_(z) of the erroneous field can now be determined from theplotted diagram as it equals the distance between B_(z) and B_(zc),since B_(zc) =B_(z) -M_(z).

After thus having determined the magnitude B_(zc) of the axial componentof the magnetic field at the location of the instrument 5 the azimuth ofthe borehole is calculated on the basis of known formulas using thecorrected values B_(xc), B_(yc), B_(zc).

It is observed that the sensor package may be included in the drillstring in various ways. The package may be suspended in the drill stringby means of a wireline and locked to the non-magnetic sections as shownin prior art, wherein the signals produced by the sensors aretransmitted to the surface via the wireline. The package may also befixedly secured to the drill string or dropped to a selected locationinside the drill string, wherein the signals produced by the sensors areeither transmitted to the surface via a wireless telemetry system orstored in a memory assembly and then read out after retrieval of thedrilling assembly from the borehole.

Furthermore, it will be appreciated that instead of plotting thediagrams shown in FIGS. 2 and 4 computerized calculations procedures maybe used to determine said corrected components B_(xc), B_(yc) and B_(zc)of the magnetic field.

Moreover, as will be explained with reference to FIGS. 5 and 6 correctedcross-axial values B_(xc) and B_(yc) for the cross-axial components ofthe measured magnetic field can be obtained in an inclined borehole witha survey instrument comprising a single magnetometer. In the embodimentshown in FIG. 5 the survey instrument includes a single magnetometer andtwo mutually orthogonal accelerometers which are all arranged in asingle plane cross-axial to the longitudinal axis of the drill string.The accelerometers are oriented along mutually orthogonal axes x and y,and the magnetometer axis m is parallel to the x-axis accelerometer. Asillustrated in FIG. 5 the magnetic field component B_(mx) measured bythe magnetometer equals the sum of the x-component B_(ox) of the earthmagnetic field B_(o) and the x-component M_(x) of the erroneous field Mcaused by drill string magnetization. When the drill string is rotatedin the borehole the magnetometer, which is stationary relative to thedrill string, reads a constant magnetic field contribution M_(x) forevery gravity high-side angle φ as determined with the x-axis and y-axisaccelerometers. In addition, the magnetometer simultaneously reads asinusoidal varying magnetic field contribution B_(ox) of the earthmagnetic field B_(o). When the drill string is rotated over about 360degrees relative to the longitudinal axis of the inclined borehole, themagnetometer reads as illustrated in FIG. 6 a sinusoidal varyingmagnetic field with amplitude B_(xyc) and zero offset M_(x) versus thegravity high-side angle φ. For a selected angular orientation of thedrill string in the borehole and consequently a selected gravityhigh-side angle φ₁, B_(xc) is obtained by correcting the magnetometerreading for the zero-offset M_(x). B_(yc) is subsequently obtained fromthe diagram shown in FIG. 6 by correction of the magnetometer readingfor zero offset M_(x) at a gravity high-side angle 90 degrees away fromthe selected orientation of the drill string.

What is claimed is:
 1. A method for eliminating the influence of drillstring magnetization on an azimuth measurement made by a magnetometerpackage fixedly mounted to a drill string, said methodcomprising:rotating the drill string; measuring a cross-axial componentB_(m) of a magnetic field B for various rotational orientations of thedrill string; determining a portion of the measured cross-axialcomponent B_(m) produced by a cross-axial drill string magnetizationM_(xy) ; utilizing the cross-axial drill string magnetization M_(xy) toeliminate its influence on azimuth measurements.
 2. The method of claim1, wherein the components B_(x), B_(y) and B_(z) of the magnetic fieldB_(m) in three mutually orthogonal directions x, y and z are measured,and wherein the influence of the cross-axial error components M_(x) andM_(y) of the drill string magnetization on the measured magnetic fieldis determined in a diagram having B_(x) as abscis and B_(y) of themagnetic field measured at various orientations of the sensor package inthe borehole are plotted.
 3. The method of claim 2, wherein the drillstring is rotated relative to the central axis z over an angularinterval of about 360 degrees, and wherein in the diagram a closedspherical curve is drawn through the cross-axial components B_(x) andB_(y) of the magnetic field measured for various orientations of thedrill string, and the cross-axial error components M_(x) and M_(y) ofthe drill string magnetization vector M are determined on the basis ofthe position of the center of the curve in the diagram.
 4. The method asclaimed in claim 3, wherein the cross-axial error components M_(x) andM_(y) of the drill string magnetization vector M are subtracted from thecross-axial components B_(x) and B_(y) of the measured magnetic field,thereby assessing corrected cross-axial values B_(xc) and B_(yc) for thecross-axial components of the measured magnetic field, and producing avector (B_(xc), B_(yc) B_(z)) corrected for the cross-axial drill stringmagnetization expressed by the formula:

    (B.sub.xc, B.sub.yc, B.sub.z)=(B.sub.x, B.sub.y, B.sub.z)-(M.sub.x M.sub.y, 0).


5. The method as claimed in claim 4, wherein the magnetometer package isprovided with three magnetometers having their axes disposedorthogonally and three gravity sensors for determining the cross-axialand axial components g_(x), g_(y), g_(z) of the local gravity vector gand wherein the portion of axial component B_(z) of the magnetic field Bproduced by the axial drill string magnetization is determined by thesteps of:calculating the gravity field strength g through: g=(g_(x) ²+g_(y) ² +g_(z) ²)^(1/2), calculating the magnetic field strength Bcorrected for cross-axial drill string magnetization through: B=(B_(xc)² +B_(yc) ² +B_(z) ²)^(1/2) and subsequently calculating a dip angle θnetween the vectors B and g through: θ=cos⁻¹ [(B_(xc) g_(x) +B_(yc)g_(y) +B_(z) g_(z) g_(z))/Bg] obtaining independently from themeasurements in the borehole the true magnitude B_(o) of the earthmagnetic field and the dip angle θ_(o) between the vectors B_(o) and gand defining in a vector diagram a cone having a central axis defined bythe gravity vector g and enveloped by B_(o), the top angle of the conebeing equal to 2θ_(o) ; representing in the same vector diagram thevector B which extends from the top of the cone at an angle θ relativeto the gravity vector g; expressing the distance E between the vector Band the base circle of the cone by the formula: E=[B² +B_(o) ² -2BB_(o)cos (θ-θ_(o))]^(1/2;) calculating E for various assumed magnitudes ofB_(z) on the basis of said formulas for B, g, θ and E and plotting in adiagram, having an abscis representing magnitudes of B_(z) and anordinate representing magnitudes of E, the various magnitudes for E thuscalculated for various magnitudes of B_(z), determining in the plotteddiagram a minimum magnitude for the distance E and assessing themagnitude of B_(z) that corresponds to the minimum magnitude for E asthe corrected magnitude B_(zc) of the axial component of the magneticfield measured by the sensor package; and determining the azimuth of theborehole on the basis of the corrected magnitudes B_(xc), B_(yc), B_(zc)of the components of the magnetic field measured by the sensor package.6. The method as claimed in claim 1, wherein the magnetometer packageincludes a single magnetometer for measuring one cross-axial componentof the magnetic field B at the location of the sensor package.
 7. Amethod for eliminating the influence of drill string magnetization on anazimuth measurement made by a magnetometer package fixedly mounted to adrill string, said method comprising:rotating the drill string over anangular interval of about 360° and measuring components B_(x), B_(y) andB_(z) of a magnetic field B in three mutually orthogonal directions x, yand z, corresponding to separate magnetometers, with the z axis as thecentral axis of the drill string; determining the position of themeasured cross-axial component B_(m) produced by the cross-axial drillstring magnetization M_(xy), by the steps comprising:defining a closedspherical curve from the measurements of B_(x) and B_(y) for variousorientations of a sensor package in the borehole where B_(x) is theabscis and B_(y) is the ordinate; determining the cross-axial errorcomponents M_(x) and M_(y) of the drill string magnetization vector M onthe basis of the position of the center of the closed spherical curve inrelation to the origin of the B_(x) and B_(y) axes; utilizing thecross-axial drill string magnetization M_(xy) to eliminate its influenceon azimuth measurements by subtracting the cross-axial error componentsM_(x) and M_(y) of the drill string magnetization M from the cross-axialcomponents B_(x) and B_(y) of the measured magnetic field, therebyassessing corrected axial values B_(xc) and B_(yc) for theaxial-components of the measured magnetic field, and producing a vector(B_(xc), B_(yc), B_(z)) corrected for the cross-axial drill stringmagnetization expressed by the formula:

    (B.sub.xc, B.sub.yc, B.sub.z)=(B.sub.x, B.sub.y, B.sub.z)-(M.sub.x, M.sub.y, 0);

determining the corrected magnitude B_(zc) of the axial component of themagnetic field by eliminating the axial component in the magnitude ofB_(z) resulting from drill string magnetization, comprising thefollowing steps:measure components g_(x), g_(y), g_(z) of the localgravity g in three mutually orthogonal directions x, y and z, with z asthe axis of the drill string and corresponding to three separate gravitymeters; calculating the gravity field strength g through: g=(g_(x) ²+g_(y) ² +g_(z) ²)^(1/2), calculating the magnetic field strength Bcorrected for cross-axial drill string magnetization through: B=(B_(xc)² +B_(yc) ² +B_(z) ²)^(1/2) ; and subsequently calculating a dip angle θbetween the vectors B and g through: θ=cos⁻¹ [(B_(xc) g_(x) +B_(yc)g_(y) +B_(z) g_(z))/Bg]; obtaining independently from the measurementsin the borehole the true magnitude B_(o) of the earth magnetic field andthe dip angle θ_(o) between the vectors B_(o) and g defining in a vectordiagram a cone having a central axis defined by the gravity vector g andenveloped by B_(o), the top angle of the cone being equal to 2θ_(o) ;representing in the same vector diagram the vector B which extends fromthe top of the cone at an angle θ relative to the gravity vector g;expressing the distance E between the vector B and the base circle ofthe cone by the formula:

    E=[B.sup.2 +B.sub.o.sup.2 -2BB.sub.o cos (θ-θ.sub.o)].sup.1/2 ;

and calculating E for various assumed magnitudes of B_(z) on the basisof said formulas for B, g, θ and E and plotting in a diagram, having anabscis representing magnitudes of B_(z) and an ordinate representingmagnitudes of E, the various magnitudes for E thus calculated forvarious magnitudes of B_(z), determining in the plotted diagram aminimum magnitude for the distance E and assessing the magnitude ofB_(z) that corresponds to the minimum magnitude for E as the correctedmagnitude B_(zc) of the axial component of the magnetic field measuredby the sensor package; and defining the azimuth of the borehole on thebasis of the corrected magnitudes B_(xc), B_(yc), B_(zc) of thecomponents of the magnetic field measured by the sensor package.
 8. Amethod for eliminating the influence of drill string magnetization in anazimuth measurement made by a magnetometer package fixedly mounted in adrill string, said method comprising:rotating the drill string;measuring a cross-axial component B_(m) of a magnetic field B forvarious rotational orientations of the drill string; determining theportion of the measured cross-axial component B_(m) produced by across-axial drill string magnetization M_(xy) to eliminate its influenceon azimuth measurements thereby assessing corrected cross-axil valuesB_(xc) and B_(yc) for the cross-axial components of the measuredmagnetic field; determining the portion of an axial-component B_(z) ofthe magnetic field B produced by an axial drill string magnetizationutiltizing three gravity sensors for determining the cross-axial andaxial components g_(x), g_(y) g_(z) of the local gravity vector g by thesteps of:calculating the gravity field strength g through: g=(g_(x) ²+g_(y) ² +g_(z) ²)^(1/2), calculating the magnetic field strength Bcorrected for cross-axial drill string magnetization through: B=(B_(xc)² +B_(yc) ² +B_(z) ²)^(1/2) ; and subsequently calculating a dip angle θbetween the vectors B and g through: θ=cos⁻¹ [(B_(xc) g_(x) +B_(yc)g_(y) +B_(z) g_(z)) /Bg]; obtaining independently from the measurementsin the borehole the true magnitude B_(o) of the earth magnetic field andthe dip angle θ_(o) between the vectors B_(o) and g and defining in avector diagram a cone having a central axis defined by the gravityvector g and enveloped by B_(o), the top angle of the cone being equalto 2θ_(o) ; representing in the same vector diagram the vector B whichextends from the top of the cone at an angle θ relative to the gravityvector g; expressing the distance E between the vector B and the basecircle of the cone by the formula:

    E=[B.sup.2 +B.sub.o.sup.2 -2BB.sub.o cos (θ-θ.sub.o)[1/2;

calculating E for various assumed magnitudes of B_(z) on the basis ofsaid formulas for B, g, θ and E and plotting in a diagram, having anabscis representing magnitudes of B_(z) and an ordinate representingmagnitudes of E, the various magnitudes for E thus calculated forvarious magnitudes of B_(z), determining in the plotted diagram aminimum magnitude for the distance E and assessing the magnitude ofB_(z) that corresponds to the minimum magnitude for E as the correctedmagnitude B_(zc) of the axial component of the magnetic field measuredby the sensor package; and determining the azimuth of the borehole onthe basis of the corrected magnitudes B_(xc), B_(yc), B_(zc) of thecomponents of the magnetic field B_(m) measured by the sensor package.9. A method for eliminating the influence of drill string magnetizationon an azimuth measurement made by a magnetometer package fixedly mountedin a drill string, said method comprising:rotating the drill string;measuring a cross-axial component B_(m) of a magnetic field B forvarious rotational orientations of the drill string; determining theportion of the measured cross-axial component B_(m) produced by across-axial drill string magnetization M_(xy) ; utilizing thecross-axial drill string magnetization M_(xy) to eliminate its influenceon azimuth measurements; determining the portion of an axial-componenetB_(z) of the magnetic field B produced by an axial drill stringmagnetization M_(z) ; and utilizing the axial drill string magnetizationM_(z) to eliminate its influence on azimuth measurements.